The effect of elemental and hydrocarbon impurities on mercuric iodide gamma ray detector performance

m__

J$.g r

r

Nuclear Inst~ments

and Methods

in Physics

Research

A 380 i 19%) 23-25

-

__

~

NUCLEAN INSTNUMENTS S MEtNODS IN NNYSICS R5!z!I~H

EI_XNER

The effect of elemental and hydrocarbon impurities on mercuric iodide gamma ray detector performance Eilene S. CrossaT*, George Buffleben”, Ed So&“, Ralph James”, Michael Schieber”, Raj Natarajanb, Vem Cerrish’ “Sandia

National ?‘exas

Laboratories,

NWear

Cul(fimia,

Technologies.

~C[Jtzst~l~ati#i~ Tech~~f~~~,

USA

USA USA

Abstract Mercuric iodide is a room temperature semiconductor material that is used for gamma ray and x-ray radiation detection. Mercuric iodide is synthesized from mercuric chloride and potassium iodide and is then purified by a series of melts and sublimation steps and by zone refining. The mercuric iodide is grown into crystals and platelets and then fabricated into detectors. Elemental contamination may be a dete~ining factor in the performance of these detectors. These contaminates may be present in the starting material or may be introduced during, or be unaffected by, the purification. growth or fabrication steps. Methods have been developed for the analysis of trace levels of elemental contamination. Inductively Coupled PIasma/Mass Spectroscopy (ICP/MS), Inductively Coupled Plasma/Optical Emission S~ctroscopy (ICPIOES) and Gas Chromatography/Mass Spectroscopy (GC/MS) are used to determine sub ppm levels of many trace elemental impurities. Trace levels of many elemental impurities in the raw mercuric iodide are significantly reduced during the pu~fication and zone refining processes. Though the levels of impnrities are reduced, poor performing mercuric iodide detectors have contamination levels remaining or reintroduced which are higher for Ag, Al, Ca. Cu, Mg. Mn, Na. Pb and Zn than detectors with good gamma ray response. This paper will discuss the analytical methodology, the effects of purification on impu~ty levels, and the correlation between detector pe~ormance and impurity levels.

1. Introduction Mercuric iodide detector performance may be influenced by trace levels of elemental impu~ties, stoichiometry and other factors. Mercuric iodide is synthesized from an aqueous solution of mercuric chloride and potassium iodide. These starting materials are either reagent grade, ultra pure (99.999~ff) or zone refined. The synthesized mercuric iodide is purified through a series of vacuum sublimations. melts, resolidifications, closed sublimations and zone refining. The purified mercuric iodide is grown into crystals or platelets and they are then fabricated into room temperature radiation detectors. Despite the impo~nce, very few publications are dedicated entirely to chemical analysis of HgIz [l-4] or mentioned in conjuncture with other properties of HgI, [5,6]. Thus, the ICP/MS was applied to HgI, [I] and it was proven that a few elements e.g. Ni are not decreased by the many purification steps used to grow the single crystals of HgI,. On the other hand, it is known that doping with Ni introduces a trapping level [7]. Bench et al. [2] applied particle induced x-ray emission, PIXE. and *Corresponding Oi48-~2/96/$15.~ PI1

author. Copyright

SO~68-9002(96)00296-~

01996

proved.that Fe, Ni. Cu and Cd are easily identitied in HgI, detectors. It is known that PIXE detects only large concentrations of above IO ppm of impurities. Hydrocarbons and total oxidized carbon was first applied by Muheim et al. [5,6], and hydrocarbons of up to Cz, were found in HgIz. A review of impurity analyses performed by spark source mass spectroscopy was published by Nicolau and Andreani (81 and showed that impu~ties of hydrocarbons determined by various laboratories may vary by several orders of magnitude. The present paper wiI1 report refinements of the analytical method and the latest results obtained by ICP/OES and ICP/MS for metal impurities and CC/MS for hydrocarbon analysis.

2. Experimental 2.1. ICPIOES and 1CPlMS ICPlOES and ICPlMS are used as compIement~ techniques. ICPlMS is the more sensitive technique with detection limits for most elements in the low parts per billion range. ICP/MS wouid be used exclusively. however

Elsevier Science B.V. All rights reserved I(a). VARIOUS

MATERIALS

ES. Cross et al. I Nucl. Instr. and Meth. in Phys. Res. A 380 (1996) 23-25

24

Table 1 Elements for which quantified results were obtained Ag Al Ca

Cr Cu CS

Mg

Na Ni Pb

CO

Fe

Mn

Zn

Ga Li

this technique is subject to several unavoidable spectral interferences that occur due to matrix/solvent effects. ICP/ OES is used to analyze these affected elements (copper and iron). In order to minimize the matrix/solvent effects, mercuric iodide is digested in a modified aqua regia solution (5:2.8 ratio of HNO,:HCI respectivety). Previously [I], KI was used as the solvent for mercuric iodide. KI contains unacceptably high impurity levels. The impurity levels detected in mercuric iodide with the aqua regia method are higher than seen with the KI dissolution method suggesting that KI is not effective at dissolving particulate contaminants from within the crystal structure. For these reasons, we now use the aqua regia method exclusively. Standards, spiked solvent, spiked mercuric iodide control samples, and mercuric iodide samples are run on a Fisons VG Plasma Quad ICPiMS and then on a Perkin Elmer Model #PlOOO ICPIOES. The data are control blank subtracted and adjusted to reflect the concentrations present in the solid samples. The error calculations vary by element, but generally the error is between 5-15%. Quantified results are obtained for the list of elements in Table 1. 2.2

Hydrocarbon analysis

Samples of 5-6 g of mercuric iodide are added to 30 ml 1 molar KI solution and 3 ml of pentane. The samples are then placed in an ultrasound bath until dissolved (about 1 week). The pentane is extracted, placed into a clean vial and allowed to evaporate to dryness in order to concentrate the samples. The samples are then reconstituted with 150 pJ of pentane and injected into the GUMS. A Hewiett Packard 5890 Gas Chromatograph with a 5970 Mass Selective Detector was used to analyze the samples. A typical run setting has the injection port and transfer line maintained at 280°C. The oven started at 30°C

Front

14 Itear

Fig. 2. Effect of zone refining on elemental iodide.

impurities in mercuric

for 5 min and then was increased by IO C”/min until it reached 28O”C, where it remained for 5 min. Splitless injections were used with a 2 min purge time and a 3 min solvent delay.

3. Sample selection

3.1. Hydrocarbon analysis The samples available for hydrogen analysis were limited to raw syn~esiz~ mercuric iodide and several steps in the purification scheme. 3.2. ICPIOES

and ICPIMS analysis

Potassium iodide: Several samples of KI (both reagent grade and ultra high purity) from different vendors, as well as zone refined KI, were analyzed in order to study the impu~ty concen~ations in the starting material. Mercuric iodide: Raw samples, samples from various steps in several purification schemes, zone refined material, doped samples and characterized detectors of mercuric iodide were selected for trace elemental analysis.

4. Analytical results

ml

Fig. 1. Effect of pu~fication

on doped mercuric

iodide.

Potassium iodide: The total elemental impurity concentration in the analyzed samples of potassium iodide ranged from 3 ppm for a sample of Johnson Mathey Puratronic Grade KI to over 26 ppm for the 99.999% Johnson Mathey KI sample. The reagent grade KI from Deepwater contained 18 ppm total elemental impurities. The concentration of impurities varied greatly between different lots from the same vendor. Na, Al, Ca, Ni and Fe were the most abundant impurities found.

Zone refining of the KI was effective at concentrating Cd, Cu. Na, and Ni at the front of the tube. Pb was concentrated in the tail end of the tube. There was no discernible effect on the distribution of Al, Ag, Ca. Cr. Fe, Mg or Mn in the zone refined tube. 4. I. Mrrt~lrric. iodide

Ag. Al. Ca, Cd. Cr, Cu. Fe, Mn. Na, Ni, Pb and Zn are typical trace level contaminates in raw synthesized mercuric iodide. Contamination levels can be as high as 6 ppm for the most abundant elements. Previously. it has been reported 1II that purification schemes can reduce the levels of most contaminates by 50%. As our data base on puritication results expands. we have observed that contamination levels in the raw samples can decrease by the last purification step, by an order of magnitude for Ca, Cr. Cu. Mn and Ni. In order to further evaluate the effectiveness of purification schemes on mercuric iodide, a series of mercuric iodide samples were doped with known concentrations of Ag. Al. Ca. Cd. Cr. Cu. Fe. Mg, Mn, Na. Ni. Pb and Zn. These samples were purified using several different methods and samples of the doped and puritied mercuric iodide as well as the residue from the purification methods were analyzed. All of the puritication methods were effective at removing or reducing the concentrations of these elements in the doped samples with very high levels present in the residue (Fig. I L CC/MS analysis identified the presence of many hydrocarbons, the ma.jor ones being straight chain Cl g-C32 hydrocarbons. The levels of hydrocarbons increased with successive purification steps that included the addition of polyethylene. The hydrocarbons detected in the mercuric iodide samples arc the same as those found in the polyethylene used in purification. The levels of hydrocarbons remained unchanged for raw mercuric iodide that was puritied without the addition of polyethylene.

at 662 keV. Many elemental impurities were found in the detectors. Fe. Zn, Cr and Al are the most abundant with concentrations between 0.5-3 ppm, Poor detectors had higher elemental contamination levels for Ag. Al. Ca. Cu. Mg. Mn. Na. Pb and Zn. Na. Pb and Zn were more than a factor of 3 higher in the poor detectors.There is no correlation hetween detector performance and Cd. Cr. Fe or Ni.

5. Conclusions ICPlMS, ICP/OES and CC/MS are effective techniques for measuring trace levels of elemental impurities. We have identified the elemental impurities that play a major role in influencing detector performance. The purity of the source materials and the effectiveness of purification schemes are vital in reducing the concentration of most trace elemental contaminates in mercuric iodide detectors. It will be possible to predict detector performance by utilizing these analysis techniques on source material.

References ES.

Cross. E. Mro~ and J.A. Olivares.

302 (1993)

MRS

Symp. Proc.

61.

G.S. Bench. A.J. Antolah. D.H. Morse, DW. Heiniken. Pontau. R.B.

James.

D.C.

David,

A.

A.E.

Burger and L. Van-

DenBerg, MRS Symp. Proc. 307 ( 1993) 67. S. Steinberg. I. Kaplan, M. Schieber. C. Ortale. N. Sktnner and L. VanDenBerg.

Nucl.

Instr. and Meth.

A 7X3 (19X9)

123. E. Soria and R.B. James, unpublished. J.T. Muheim, T. Kobayashi and E. Kaldis, Nucl. Instr. and

Samples of mercuric iodide were obtained at several intervals along a zone refined tube. The highest levels of elemental contamination were found in the front (i.e. the last to freeze) section of the tube for the following elements: Ag. Al, Ca. Cu. Fe, Mg, Mn, Na, Ni and Zn. There is no apparent segregation of Pb. Fig. 2 details the profile of contamination levels along a zone refined tube of mercuric iodide for Ag. Fe. Na and Zn.

Meth. 243

( 1983)

39.

M. Poiechotka and E. Kaldis, Nucl. Instr. and Meth. A 2X3 (1989)

III.

M. Schieber, H. Hermon and M. Roth. MRS 302 (1993) M.

Symp. Proc.

347.

Schieber,

R.B. James and T.E.

ductors for Room Temperature

Schlesinger,

Nuclear

Detector

SemtconApplica-

tions. eds. T.E. Schlesinger and R.B. James. Academtc Press Serves on semi-conductors and Semi-metals

1995. Vol. 33, p.

S61.

4.1.3. Drtec’tor pet~fi~rmawe Several detectors were defabricated and analyzed for elemental impurities. Half exhibited good gamma ray response and the other half of the samples exhibited bad gamma ray response. This response is expressed by calculating a quality factor for each detector as follows:

H. Hermon.

M. Schieber and M. Roth, these Proceedings

(9th Int. Workshop on Room Temperature Semiconductor Xand y-Ray

Detectors, Associated Electronics

and Applica-

tions, Grenoble, France. 1995) Nucl. Instr. and Meth. A 3X0 (1996)

IO.

Y.F. Nicolau and A.M. Andreani. J. Cryst. Growth X7

( 198X)

117.

I(a).

VARIOUS

MATERIALS